This invention generally relates to the field of metallurgy. More particularly, this invention relates to the production of rapidly solidified ingots.
It is well known that the properties of a metal or alloy can be affected by the cooling rate used to solidify the metal or alloy. Currently, there is a great interest in the development of alloys having uniformly dispersed, fine particles and a uniformly fine grained microstructure. It is known that rapid solidification can produce a uniformly dispersed and fine grained microstructure with a minimum amount of chemical segregation. Rapid solidification allows for massive phases to be eliminated from the alloy and for the solubility of the alloying elements to be increased. Such materials characteristically have high tensile, fatigue and creep strength. The fine grain size may also enable the materials to undergo enormous tensile elongations at elevated temperatures without experiencing fracture. This property, known as superplasticity, is obtained after extensive thermomechanical processing of the material containing uniformly dispersed particles, which produces a fine grain recrystallized microstructure.
In most cases, the uniform dispersion of fine intermetallic particles (200-500 nm in size) is not present in the alloy immediately after rapid solidification. The typical microstructure immediately after the rapidly solidification of the alloy is a fine, dendritic cast grain structure. The object of rapid solidification is to hold the constituents in solution, enabling them to precipitate out in a uniform manner during a subsequent heat treatment step. For certain alloys, the precipitation process may be partially completed during the rapid solidification step. A uniformly dispersed, fine precipitate structure is a prerequisite for developing a fine grain recrystallized (or recovered) microstructure during a subsequent thermomechanical processing operation. In the discussion which follows, for simplicity, the terms fine grain microstructure, finely dispersed microstructure, etc. are used interchangeably since both are simultaneously present in the thermomechanically processed material.
Rapid solidification and the production of uniformly distributed fine particles or dispersoids, however, is not trivial. Typically, the alloying elements are in solution when the alloy is in a molten state. During conventional casting, solidification rates of only 0.1.degree.-1.degree. K./s may be achieved. These solidification rates are insufficient to maintain the alloying elements in solution, which is necessary for producing a fine dispersion in the molten alloy and, as a result, insoluble intermetallic compounds precipitate out as coarse particles in the solidified ingot. The key to maintaining a fine dispersion of particles and an improved chemical homogeneity is rapid solidification, preferably in the range of 10.degree.-1,000.degree. K./s, of the molten alloy.
Several techniques have been developed to rapidly cool molten alloys and to thereby achieve the improved properties mentioned above. One prior art process produces rapidly cooled powders and then utilizes powder metallurgy processes, such as hot pressing or hot isostatic pressing, to consolidate the powders into a billet. According to methods of this type, the powders are made using a gas jet or a rotating spinner to atomize a stream of molten metal. The metal particles are rapidly solidified by a gas quenching medium to produce a chill cast powder. The cooled powder must then be consolidated to form a mill product suitable for fabrication into parts. This consolidation process might require some or all of the following: sizing of the powder, cold pressing of the powder, vacuum degassing, canning, hot compacting and other steps designed to form a dense product without introducing oxides, gases and other contaminants into the product.
Another known method of rapid solidification is melt spinning or melt extraction to produce rapidly solidified ribbons which then are consolidated into a workable product. Still another method involves spray forming and then depositing the spray droplets so as to form a solid ingot.
In one of the spray forming methods of rapid solidification, a molten alloy is allowed to flow onto a hot, spinning disc. The hot spinning disc atomizes the liquid and propels the tiny molten droplets outward to "splatter" against a water cooled mold where they rapidly solidify. The mold may move up and down relative to the spinning disc so that the droplets are spread along its inner surface. Movement of the mold is timed so that each layer is solidified before the next layer is deposited. Layers are repeatedly deposited until an ingot of a suitable thickness has been formed. This method is more cost effective then the two former methods because a solid product is obtained in a single step without having to consolidate the powder or pulverize ribbon materials.
The cost of spray formed materials is still greater than that of conventional or continuous casting processes. The reasons for the increased cost include the expense of the gas used in the atomization process, the high handling cost of the gas, the additional maintenance cost of the atomization nozzle and related structures, cost increases related to the subdivision of liquid into a spray which slows down the production process and makes the process more cumbersome. Furthermore, gas entrapment in the solid product can be a problem.
With the limitations of the prior art in mind, it is an object of the present invention to provide a method and apparatus for producing rapidly solidified ingots having uniformly distributed dispersoids and a fine scale microstructure.
This invention also seeks to provide an apparatus and process in which a rapidly solidified ingot is continuously cast in a one step process from a molten metal. As such, the present invention is a continuous process in that it forms an ingot without first dividing the melt in small, individual particles for solidification purposes and then recombining the solidified particles into an ingot form. This makes the invention a very cost effective method for producing rapidly solidified ingots.
In achieving the above objects, the present invention provides for a molten alloy to be held in a heated crucible having an opening through which the molten alloy flows into a chilled mold during production of the ingot. The mold itself is tilted at an angle relative to the pouring stream of the melt and is also rotated about this inclined axis.
As the molten alloy is being poured into the mold, it contacts the chilled mold surfaces and is sheared from the molten stream. The thin, sheared layer of the stream flows along the mold surface generally downward under the influence of gravity and sideways under the influence of the mold's rotation, in other words helically, toward the bottom of the mold. Depending on the rate at which the molten alloy is poured into the mold, the rotational speed of the mold is adjusted so that the sheared layer being deposited on the mold surfaces results in a thickness of about 0.2 mm. Also, the mold height and the pouring rate of liquid alloy must be controlled so that the alloy superheat is not lost significantly before the liquid contacts the mold surface, in other words, so that the liquid alloy does not partially solidify during the actual act of pouring.
The chilled mold and the thinness of the deposited alloy layer leads to a conduction dominated freezing of the sheared alloy and therefore a very high cooling rate. Depending on the pouring rate of the molten stream and the rotational speed of the mold, the inclination of the mold can be varied to ensure that the sheared alloy reaches the bottom of the mold before completely solidifying. To maintain the high cooling rate, as the bottom of the mold begins to fill-up with the solidified ingot, the inclination angle can also be varied during the course of melt deposition. Another possible approach to filling the mold would involve a slow horizontal displacement of the inclined mold to regulate the location where the molten stream hits the inside surface of the mold, or previously solidified alloy, thereby enabling the mold to be filled from the bottom to the top, as well radially inward and a combination thereof. Yet another approach may involve lowering the mold along the inclined axis, to allow the stream to contact a location higher up on the inside mold surface. This method would effectively achieve the same results as generally horizontally moving the mold. In another application, such as during use of large diameter molds, repeated to-and-fro horizontal motion may be required to allow the mold to completely fill from the bottom to top. After an ingot has been formed, it is removed from the mold so that it may be further worked, if needed, to achieve the desired fine grain recrystallized, or recovered, microstructure.
Since the rotation of the mold causes the molten alloy to be deposited in a thin, sheared layer, heat can be extracted at an extremely rapid rate causing the molten alloy to freeze without permitting insoluble intermetallic compounds to form and precipitate out as coarse particles in the solidified ingot. With the anticipated cooling rate of the melt being in the range of 1000.degree. K./s, the alloying elements are held in solution and lead to a uniform dispersion of fine particles either directly after solidification or after a subsequent aging treatment. The resulting ingot will have a uniformly fine grained microstructure, in the as-cast condition and/or after suitable thermomechanical processing.
As can be seen from the brief discussion presented above, various designs for the mold and the mechanism for tilting, rotating and displacing the mold, relative to the poured stream of the molten alloy, are envisioned and within the purview of this invention. Furthermore, additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.